Ensure all mistral projects are ready for i18n and l10n

Add oslo_i18n support to mistral
Currently user-facing strings in mistral are not marked for translation.
We should ensure, that we use oslo_i18n to mark all user-facing strings for translation
We should ensure, that we use correct tools (for example horizon switched from makemessages to babel and django-babel, to facilitate js translation)
We should ensure, that every mistral-project is ready to be translated
We should integrate mistral translation efforts with zanata https://translate.​openstack.​org and allow easy and automatic translation imports.